Dynamics of Salt Secretion by Sporobolus spicatus(Vahl) Kunth from Sites of Differing Salinity

Secretion of salts by bicellular salt glands and the water relationsof the grass Sporobolus spicatus were investigated at four sitesalong the coast of the Red Sea in Egypt that differed in theextremity of salinity and drought. Salt eliminated by the leaveswas similar in its composition at all sites. Na⁺and Cl^-werethe dominant ions in the soil, and together comprised about93% of the dry weight of secreted salt. The molar ratio of K⁺:Na⁺inthe plant leaves was more than ten-fold that in the interstitialsoil solution and thirteen-times that in the secreted salts,reflecting the high selectivity of the secretion mechanism forNa⁺. The concentration of Na⁺in the solution transported tothe leaves between 0900 and 1500 h was less than 0.1% of thatin the soil solution. Accumulation of salts by the plant shoots,which increased with increasing soil salinity and drought, wasmaximal during the day when the extent of secretion greatlyreduced. The ionic osmotic potential (